The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, planarization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a moving polishing pad, such as a circular pad or linear belt. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened or soft surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-active agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
A reoccurring problem in CMP is non-uniform polishing. Due to a variety of factors, some portions of the substrate tend to be polished at a different rate than other parts of the substrate. This non-uniform polishing can occur even if a uniform pressure is applied to the backside of the substrate. In addition, a substrate arriving at the polishing apparatus may have an initial thickness that is non-uniform. Therefore it is desireable to provide a carrier head that can apply different pressures to different regions of the substrate during chemical mechanical polishing to compensate for non-uniform polishing rates or for non-uniformity in the initial thickness of the substrate.
An example of non-uniform polishing is the so-called xe2x80x9ccenter fast effectxe2x80x9d, i.e., the tendency of the central region of the substrate to be polished faster than the outer region of the substrate.
In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier structure, a first flexible membrane extending below the carrier structure, and a plurality of chambers between the first flexible membrane and the carrier structure. A bottom surface of the flexible membrane provides a substrate-mounting surface. The plurality of chambers are configured to apply a first pressure to a substrate in an annular loading area having an inner diameter, and the plurality of chambers permit control of the first pressure applied to the substrate in the loading area and the inner diameter of the annular loading area.
Implementations of the invention may include one or more of the following features. The plurality of chambers may be configured to apply a second pressure to the substrate in a central loading area surrounded by the annular loading area. The second pressure may be less than the first pressure. A second flexible membrane may be positioned between the first flexible membrane and the carrier structure. The second flexible membrane may include a first membrane portion which can be brought into contact with an inner surface of the first flexible membrane, and a second membrane portion may be connected to a central section of the first membrane portion and define a first chamber. Evacuation of the first chamber may draw the second membrane portion upwardly and may pull the central section of the first membrane portion away from first flexible membrane to increase an inner diameter of an annular section of the first membrane portion that contacts the first flexible membrane. A third membrane portion may be connected to an edge section of the first membrane portion and may define a second chamber. Evacuation of the second chamber may draw the third membrane portion upwardly and may pull the edge section of the first membrane portion away from first flexible membrane to reduce an outer diameter of the annular section of the first membrane portion in contact with the first flexible membrane. The first flexible membrane may include an outer membrane portion to contact the substrate and an inner membrane portion joined to a central section of the outer membrane portion and defining a first chamber. Evacuation of the first chamber may draw the inner membrane portion upwardly and may pull the central section of the outer membrane portion away from the substrate to increase an inner diameter of an annular section of the outer membrane portion that contacts the substrate. Pressurization of the second chamber may push the inner membrane portion outwardly to contact the first membrane portion. There may be a fluid connection to a volume between the central section of the outer membrane and the substrate.
In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier structure, a first flexible membrane having a perimeter portion connected to the carrier structure and a central portion with a lower surface that provides a substrate mounting surface, and a second flexible membrane having a central portion secured to the carrier structure, a perimeter portion secured to the carrier structure, an annular flap secured to the carrier structure, and a middle portion having a lower surface that contacts an upper surface of the central portion of the first flexible membrane in an annular region. A first volume between the first flexible membrane and the second flexible membrane provides a first chamber, a second volume between the second flexible membrane and the carrier structure inside the annular flap provides a second chamber, and a third volume between the second flexible membrane and the carrier structure between the annular flap and the perimeter portion provides a third chamber.
Implementations of the invention may include one or more of the following features. The first, second and third chambers may permit control of a pressure applied to the substrate in the annular region and control of an inner diameter and an outer diameter of the annular region. Pressurization of the first chamber may push the middle portion of the second flexible membrane away from the first flexible membrane to increase the inner diameter of the annular region, whereas evacuation of the first chamber may pull the middle portion of the second flexible membrane toward from the first flexible membrane to decrease the inner diameter of the annular region. Pressurization of the second chamber may push the middle portion of the second flexible membrane toward the first flexible membrane to decrease the inner diameter of the annular region, whereas evacuation of the second chamber may pull the middle portion of the second flexible membrane away from the first flexible membrane to increase the inner diameter of the annular region. Pressurization of the third chamber may push the middle portion of the second flexible membrane toward the first flexible membrane to increase the outer diameter of the annular region, whereas evacuation of the third chamber may pull the middle portion of the second flexible membrane away from the first flexible membrane to decrease the outer diameter of the annular region. The central portion of the first flexible membrane may have an aperture, and a clamp may extend through the aperture to secure the first flexible membrane to the carrier structure. The clamp may include a passage to fluidly connect the first chamber to a pressure source.
Potential advantages of implementations of the invention may include zero or more of the following. Both the pressure and the loading area of the flexible membrane against the substrate may be varied to compensate for non-uniform polishing. The carrier head may apply pressure to the substrate in an annular loading area, and both the inner diameter and the outer diameter of the annular loading area may be controlled. The carrier head may either increase or decrease the pressure at the substrate center relative to the pressure on other portions of the substrate. Thus, non-uniform polishing of the substrate, such as the center-slow effect, may be reduced, and the resulting flatness and finish of the substrate may be improved.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.